Low-friction sliding member in transmission, and transmission oil therefor

ABSTRACT

A low-friction sliding member in a transmission is at least one of sliding members which are in slidable contact with each other in presence of a transmission oil composition. The low-friction sliding member includes a base material having a sliding section. A hard carbon thin film is coated at a sliding surface of the sliding section of the base material. The hard carbon thin film contains hydrogen in an amount not more than 20 atomic %. The transmission oil composition contains at least one of organic oxygen-containing compound and aliphatic amine compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation based upon U.S. patent application Ser. No. 10/921,291, filed Aug. 19, 2004, which claims priority from Japanese Patent Applications No. P2003-208366, filed Aug. 22, 2003, No. P2003-208368, filed Aug. 22, 2003, No. P2004-230038, filed Aug. 6, 2004 and No. P2004-230039, filed Aug. 6, 2004, the entire contents of which are incorporated herein by reference, and which has the following related applications: U.S. patent application Ser. No. 09/545,181, filed on Apr. 7, 2000, now U.S. Pat. No. 6,844,068, which is based on Japanese Patent Application No. Hei-11-102205 filed Apr. 9, 1999; Ser. No. 10/468,713, filed on Sep. 27, 2002, now U.S. Pat. No. 7,086,362, which is the national stage (United States) application of PCT Application JP02/10057, based on Japanese Patent Application No. 2001-117680, filed Apr. 17, 2001; Ser. No. 10/355,099, filed Jan. 31, 2003, now U.S. Pat. No. 6,806,242, which is based on Japanese Patent Application No. 2002-45576, filed Feb. 22, 2002; Ser. No. 10/682,559, filed Oct. 10, 2003, now U.S. Pat. No. 6,886,521, which is based on Japanese Patent Application No. 2002-302205 filed Oct. 16, 2002; and Ser. No. 10/692,853, filed on Oct. 27, 2003, now U.S. Pat. No. 6,969,198, which is based on Japanese Patent Application No. 2002-322322 filed Oct. 16, 2002.

BACKGROUND OF THE INVENTION

This invention relates to improvements in a low-friction sliding member in a transmission, and more particularly to the low-friction sliding member suitable to be used in a continuously variable transmission and in an automatic transmission and a transmission oil composition used for the low-friction sliding member.

Global environmental problems such as global warming and ozone layer destruction are coming to the fore. As it has been said that the global warming is significantly affected by CO₂ emission, the reduction of CO₂ emission, notably the setting of CO₂ emission standards, has become a big concern to each country. One of challenges to reduce CO₂ emission is to improve a fuel efficiency or fuel economy that depends on the performance of sliding members and a lubricating oil applied thereto. It will be understood that the sliding members and the lubricating oil perform great roles for improving the fuel economy.

The sliding members have been required to be excellent in wear resistance even in a sliding site where a frictional wear circumstance is severe as compared with other sliding sites in the transmission. Recently, application of a variety of hard thin films has been in progress. In general, hard carbon thin films have been expected as low-friction materials for the sliding members because they have low friction coefficients in air and in presence of lubricating oil as compared with wear-resistant hard coating materials such as titanium nitride (TiN) or chromium nitride (CrN).

Additionally, there are the following approaches to improve the fuel economy in connection with the lubricating oil: (1) lowering the viscosity of the lubricating oil, thereby reducing viscous resistance in hydrodynamic lubrication regions and agitation resistance in the engine; and (2) adding a suitable friction modifier and other additives into the lubricating oil so as to reduce friction losses under the conditions of mixed lubrication and boundary lubrication.

Such approaches concerning frictional characteristics in connection with the lubricating oil are discussed in Japan Tribology Congress 1999. 5, Tokyo, Proceeding Page 11-12, KANO et al and in World Tribology Congress 2001. 9, Vienna, Proceeding Page 342, KANO et al.

SUMMARY OF THE INVENTION

However, it has also been revealed that general hard carbon thin films excellent in low frictional characteristics in air are not necessarily high in friction reduction effect in presence of a lubricating oil, and that such hard carbon thin films cannot sufficiently exhibit the friction reduction effect even if a lubricating oil (composition) containing known an organomolybdenum compound is applied for the general hard carbon thin films.

It is, therefore, an object of the present invention to provide an improved low-friction sliding member in a transmission and an improved transmission oil composition therefor, by which drawbacks encountered in conventional techniques can be effectively overcome.

Another object of the present invention is to provide an improved low-friction sliding member in a transmission and an improved transmission oil composition therefor, by which the sliding member has an extremely excellent low frictional characteristics and is excellent in wear resistance while stably exhibiting the low frictional characteristics for a long time.

An aspect of the present invention resides in a low-friction sliding member in a transmission which sliding member is at least one of sliding members which are in slidable contact with each other in presence of a transmission oil composition. The low-friction sliding member comprises a base material having a sliding section. A hard carbon thin film is coated at a sliding surface of the sliding section of the base material. The hard carbon thin film contains hydrogen in an amount not more than 20 atomic %. Here, the transmission oil composition contains at least one of organic oxygen-containing compound and aliphatic amine compound.

Another aspect of the present invention resides in a transmission oil composition used for a low-friction sliding member in a transmission which sliding member is at least one of sliding members which are in slidable contact with each other in presence of the transmission oil composition. The low-friction sliding member includes a base material having a sliding section, and a hard carbon thin film coated at a sliding surface of the sliding section of the base material. The hard carbon thin film contains hydrogen in an amount not more than 20 atomic %. The transmission oil composition comprises at least one of organic oxygen-containing compound and aliphatic amine compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary sectional view of a tooth of a gear used in a continuously variable transmission, as an embodiment of a low-friction sliding member according to the present invention; and

FIG. 2 is a fragmentary sectional view of a tooth of a gear used in an automatic transmission, as another embodiment of a low-friction sliding member according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is discussed below in detail. In the following description, all percentages (%) are by mass unless otherwise specified.

First Embodiment

A first embodiment of a low-friction sliding member in a transmission, according to the present invention will be discussed.

A low-friction sliding member in a continuously variable transmission (CVT) is at least one of sliding members which are in slidable contact with each other in presence of a continuously variable transmission oil or fluid (composition) which may be referred to as “CVTF (composition)”. The low-friction sliding member includes a base material formed of, for example, a carburized SCM 420 steel. The SCM steel is according to JIS (Japanese Industrial Standard) G 4105. The base material has a sliding section whose sliding surface is coated with a hard carbon thin film. The continuously variable transmission oil (may be referred to as “lubricating oil”) contains at least one of organic oxygen-containing compound and aliphatic amine compound as ashless friction modifier(s). By virtue of this continuously variable transmission oil, at least one of the sliding surfaces of the sliding members is coated with the hard carbon thin film, and therefore the sliding members can be in slidable contact with each other with an extremely low friction as compared with that in conventional techniques. Examples of the low-friction sliding member are gears, rotatable shafts, parts of bearings, and the like in the continuously variable transmission. FIG. 1 shows a part of a tooth 10 of a gear used in the continuously variable transmission. The gear tooth 10 includes the base material 12 having the sliding surface which is coated with the hard carbon thin film 14.

The hard carbon thin film is formed of, for example, DLC (diamond-like carbon) material which is mainly constituted of carbon atom and amorphous. The DLC material takes a diamond structure (SP³ bonding) and/or a graphite structure (SP² bonding) in bonding mode among carbons. More specifically, the hard carbon (DLC) thin film is formed of hydrogen-free amorphous carbon (a-C) that consists of carbon, hydrogen-containing amorphous carbon (a-C:H), or metal carbide or metal carbon (MeC) that contains as a part a metal element of titanium (Ti) or Molybdenum (Mo).

As a hydrogen content in the hard carbon thin film increases, the friction coefficient increases. Accordingly, in the present invention, it is necessary that the upper limit of the hydrogen content in the hard carbon thin film is 20 atomic %. In order to sufficiently lower the friction coefficient in the lubricating oil during sliding and to obtain a stable sliding characteristics, the hydrogen content in the hard carbon thin film is preferably not more than 10 atomic %, and more preferably not more than 0.5 atomic %.

The hard carbon thin film having such a low hydrogen content is obtained by a PVD or CVD process that substantially does not use hydrogen and/or hydrogen-containing compound, such as a sputtering process or an ion plating process. In this case, it is preferable to carry out a film-forming operation for the hard carbon thin film upon baking of a reactor and tools for supporting the base material and upon sufficiently cleaning the surface of the base material in order to reduce the hydrogen content in the hard carbon thin film, in addition to using gas containing no hydrogen during the film-forming operation.

The hard carbon thin film is formed on the surface of the base material. Since the film thickness of the hard carbon thin film is considerably small, the surface roughness of the base material in a condition before the hard carbon thin film is formed largely affects the surface roughness of the hard carbon thin film formed on the surface of the base material. Accordingly, the surface roughness (center line average roughness) Ra of the base material is preferably not larger than 0.1 μm in the condition where the base material has not yet been coated with the hard carbon thin film. If the surface roughness of the base material exceeds 0.1 μm, projecting portions due to the surface roughness of the hard carbon thin film increases a local Hertz's contact pressure to an opposite member, thereby resulting in induction of formation of crack in the hard carbon thin film. The surface roughness Ra is explained as R_(a75) in JIS (Japanese Industrial Standard) B 0601 (:2001).

The hard carbon thin film preferably has a surface hardness or micro Vickers hardness (Hv) ranging from 1000 to 3500 as measured under application of a 10 g load and a thickness ranging from 0.3 to 2.0 μm. If the surface hardness and thickness of the hard carbon thin film are out of the above ranges, wear of the hard carbon thin film will occur in case that the surface hardness (Hv) is smaller than 1000 and the thickness is smaller than 0.3 μm, whereas peeling-off of the hard carbon thin film tends to occur in case that the surface hardness (Hv) exceeds 3500 and the thickness exceeds 2.0 μm.

The low-friction sliding member in the continuously variable transmission is particularly effective to be used as sliding members of a continuously variable transmission which sliding members have respective sliding surfaces which are in slidable contact with the sliding surfaces of the opposite members which are formed of an iron-based material or an aluminum alloy, in presence of the continuously variable transmission oil provided between the contactable sliding surfaces. The continuously variable transmission in the present invention means a transmission in which speed-changing operation is continuously made. A typical example of the continuously variable transmission has a mechanism in which fixed and movable pulley sheaves (constituting a pulley) are mounted on each of drive and driven shafts, a metallic belt being put between the pulley sheaves so that the metallic belt is passed on the both pulleys mounted on the drive and driven shafts. Accordingly, power from an engine is transmitted from the pulley on the drive shaft to the pulley on the driven shaft through the metallic belt thereby continuously making the speed-changing operation for the vehicle.

Next, the continuously variable transmission oil or lubricating oil used for the continuously variable transmission will be discussed.

The continuously variable transmission oil or lubricating oil (composition) preferably includes a base oil and at least one of the organic oxygen-containing compound and an aliphatic amine compound as ashless friction modifier. It is to be noted that an extremely excellent low friction characteristics can be exhibited in presence of the lubricating oil at the sliding surface coated with the hard carbon thin film.

The base oil is not particularly limited and can be any base oil (compound or compounds) commonly used for a lubricating oil, such as a mineral oil, a synthetic oil, an oil and fat (compound), or any combination of the mineral oil, the synthetic oil and the oil and fat.

Specific examples of the mineral oil include paraffin-based or naphthene-based oil, and n-paraffin, prepared by extracting a lubricating oil fraction from petroleum by atmospheric or reduced-pressure distillation, and then, purifying the obtained lubricating oil fraction by using at least one of the following treatments: solvent deasphalting, solvent extraction, hydrogenolysis, solvent dewaxing, hydrogenation purification, sulfuric acid treatment, clay treatment and the like which may be used in suitable combination. It is general to purify the obtained lubricating oil fraction by using hydrogenation purification or solvent purification. Additionally, it is preferable to use the mineral oil which is obtained by purifying the lubricating oil fraction using high-hydrogenolysis process which is capable of largely decreasing aromatic components, or the mineral oil produced by a process for isomerizing GTL (gas to liquid) Wax.

Specific examples of the synthetic oil include: poly-a-olefins (such as 1-octene oligomer, 1-decene oligomer and ethylene-propylene oligomer), hydrides of poly-α-olefins, isobutene oligomers, hydrides of isobutene oligomers, isoparaffins, alkylbenzenes, alkylnaphthalenes, diesters (such as ditridecyl glutarate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate and dioctyl sebacate), polyol esters (such as trimethylolpropane caprylate; trimethylolpropane pelargonate; trimethylolpropane ester such as trimethylolpropane isostearinate; pentaerythritol ester such as pentaerythritol-2-ethyl hexanoate and pentaerythritol pelargonate), polyoxyalkylene glycol, dialkyldiphenyl ether, and polyphenyl ether. Among these synthetic oil compounds, preferred are poly-α-olefins, such as 1-octene oligomer and 1-decene oligomer and hydrides thereof.

As the base oil in the lubricating oil for the continuously variable transmission, the above-mentioned mineral and synthetic oil (compounds) may be used alone, or in the form of a mixture of any two or more thereof with no limitation on the mixture ratio.

The sulfur content of the base oil is not particularly restricted. The sulfur content is preferably not more than 0.2%, more preferably not more than 0.11, much more preferably not more than 0.05%. Additionally, it is preferable to use, as the base oil, mineral oil which is purified by hydrogenation or synthetic oil because such oil has a sulfur content of not more than 0.005% or substantially no sulfur content (not more than 5 ppm).

The aromatic content of the base oil is also not particularly restricted. The aromatic content of the base oil is preferably 15% or less, more preferably 10% or less, and most preferably 5% or less in order that the lubricating oil for the manual transmission of the automotive vehicle maintains its low friction characteristics for a long time. When the aromatic content exceeds 15%, the base oil undesirably deteriorates in oxidation stability.

Herein, the aromatic content is defined as the amount of aromatics fractions determined according to ASTM D2549 “Standard Test Method for Separation of Representative Aromatics and Nonaromatics Fractions of High-Boiling Oils by Elution Chromatography”.

The kinematic viscosity of the base oil is not particularly restricted. When the lubricating oil is used for the lubricating oil for the continuously variable transmission, the kinematic viscosity of the base oil is preferably 2 mm²/s or higher, more preferably 3 mm²/s and, at the same time, is preferably 20 mm²/s or lower, more preferably 10 mm²/s or lower, most preferably 8 mm²/s or lower, as measured at 100° C. When the kinematic viscosity is lower than 2 mm²/s at 100° C., the lubricating oil can provide a sufficient wear resistance and be inferior in vaporization characteristics. When the kinematic viscosity exceeds 20 mm²/s, the lubricating oil is difficult to exhibit a low frictional characteristics and may be degraded in vaporization characteristics, which are not preferable. In connection with the present invention, at least two base oils may be freely selected to be mixed to form a mixture, in which the kinematic viscosity of the single base oil may be out of the above-mentioned range as far as the kinematic viscosity of the mixture at 100° C. falls within the above-mentioned preferable range.

The viscosity index of the base oil is not particularly restricted, and is preferably 80 or higher, more preferably 100 or higher, most preferably 120 or higher, when the lubricating oil is used for the continuously variable transmission. Increasing the viscosity index of the base oil can provide the lubricating oil for the manual transmission, excellent in low temperature viscosity characteristics and fuel economy performance.

The organic oxygen-containing compound is preferably at least one compound selected from a group consisting of mono or polyalcohols, carboxylic acids, ethers, esters and their derivatives, and its content is preferably within a range of from 0.05 to 3.0° based on the total amount of the lube oil.

Examples of alcohols are mentioned below.

1.1 Monoalcohols. 1.2 Dialcohols.

1.3 Tri and higher polyalcohols. 1.4 Alkyleneoxide adducts of alcohols selected from the above 1 to 3. 1.5 Mixtures of one or more selected from the above 1.1 to 1.4.

Monoalcohols have one hydroxyl group in the molecule, including, for example, monohydric alkyl alcohols having from 1 to 40 carbon atoms (in which the alkyl group may be linear or branched) such as methanol, ethanol, propanol (1-propanol, 2-propanol), butanol (1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol), pentanol (1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-2-butanol, 2-methyl-2-butanol, 2,2-dimethyl-1-propanol), hexanol (1-hexanol, 2-hexanol, 3-hexanol, 2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol, 3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol, 4-methyl-1-pentanol, 4-methyl-2-pentanol, 2,3-dimethyl-1-butanol, 2,3-dimethyl-2-butanol, 3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol, 2,2-dimethylbutanol), heptanol (1-heptanol, 2-heptanol, 3-heptanol, 2-methyl-1-hexanol, 2-methyl-2-hexanol, 2-methyl-3-hexanol, 5-methyl-2-hexanol, 3-ethyl-3-pentanol, 2,2-dimethyl-3-pentanol, 2,3-dimethyl-3-pentanol, 2,4-dimethyl-3-pentanol, 4,4-dimethyl-2-pentanol, 3-methyl-1-hexanol, 4-methyl-1-hexanol, 5-methyl-1-hexanol, 2-ethylpentanol), octanol (1-octanol, 2-octanol, 3-octanol, 4-methyl-3-heptanol, 6-methyl-2-heptanol, 2-ethyl-1-hexanol, 2-propyl-1-pentanol, 2,4,4-trimethyl-1-pentanol, 3,5-dimethyl-1-hexanol, 2-methyl-1-heptanol, 2,2-dimethyl-1-hexanol), nonanol (1-nonanol, 2-nonanol, 3,5,5-trimethyl-1-hexanol, 2,6-dimethyl-4-heptanol, 3-ethyl-2,2-dimethyl-3-pentanol, 5-methyloctanol, etc.), decanol (1-decanol, 2-decanol, 4-decanol, 3,7-dimethyl-1-octanol, 2,4,6-trimethylheptanol, etc.), undecanol, dodecanol, tridecanol, tetradecanol, pentadecanol, hexadecanol, heptadecanol, octadecanol (stearyl alcohol, etc.), nonadecanol, eicosanol, heneicosanol, tricosanol, tetracosanol; monohydric alkenyl alcohols having from 2 to 40 carbon atoms (in which the alkenyl group may be linear or branched and the double bond may be in any desired position) such as ethenol, propenol, butenol, hexenol, octenol, decenol, dodecenol, octadecenol (oleyl alcohol, etc.); monohydric (alkyl)cycloalkyl alcohols having from 3 to 40 carbon atoms (in which the alkyl group may be linear or branched, and the alkyl group and the hydroxyl group may be in any desired position) such as cyclopentanol, cyclohexanol, cycloheptanol, cyclooctanol, methylcyclopentanol, methylcyclohexanol, dimethylcyclohexanol, ethylcyclohexanol, propylcyclohexanol, butylcyclohexanol, dimethylcyclohexanol, cyclopentylmethanol, cyclohexylethanol (1-cyclohexylethanol, 2-cyclohexylethanol, etc.), cyclohexylmethanol, cyclohexylpropanol (3-cyclohexylpropanol, etc.), cyclohexylbutanol (4-cyclohexylbutanol, etc.) butylcyclohexanol, 3,3,5,5-tetramethylcyclohexanol; (alkyl)aryl alcohols (in which the alkyl group may be linear or branched, and the alkyl group and the hydroxyl group may be in any desired position) such as phenyl alcohol, methylphenyl alcohol (o-cresol, m-cresol, p-cresol), creosol, ethylphenyl alcohol, propylphenyl alcohol, butylphenyl alcohol, butylmethylphenyl alcohol (3-methyl-6-tert-butylphenyl alcohol, etc.), dimethylphenyl alcohol, diethylphenyl alcohol, dibutylphenyl alcohol (2,6-di-tert-butylphenyl alcohol, 2,4-di-tert-butylphenyl alcohol, etc.), dibutylmethylphenyl alcohol (2,6-di-tert-butyl-4-methylphenyl alcohol, etc.), dibutylethylphenyl alcohol (2,6-di-tert-butyl-4-ethylphenyl alcohol, etc.), tributylphenyl alcohol (2,4,6-tri-tert-butylphenyl alcohol, etc.), naphthol (α-naphthol, β-naphthol, etc.), dibutylnaphthol (2,4-di-tert-butyl-α-naphthol, etc.); 6-(4-hydroxy-3,5-di-tert-butylanilino)-2,4-bis(n-octylthio)-1,3,5-triazine, and their mixtures.

Of those, more preferred are linear or branched alkyl or alkenyl alcohols having from 12 to 18 carbon atoms such as oleyl alcohol and stearyl alcohol, in that they may more effectively lower the friction between the sliding member coated with the hard carbon thin film and the sliding surface of any other material and that they are poorly volatile and therefore may exhibit their friction-reducing effect even at high temperatures.

Dialcohols are concretely those having two hydroxyl groups in the molecule, including, for example, alkyl or alkenyldiols having from 2 to 40 carbon atoms (in which the alkyl or alkenyl group may be linear or branched, the double bond of the alkenyl group may be in any desired position, and the hydroxyl group may also be in any desired position) such as ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, neopentyl glycol, 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, 2-methyl-1,3-propanediol, 1,5-pentanediol, 1,6-hexanediol, 2-ethyl-2-methyl-1,3-propanediol, 2-methyl-2,4-pentanediol, 1,7-heptanediol, 2-methyl-2-propyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 1,8-octanediol, 1,9-nonanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,10-decanediol, 1,11-undecanediol, 1,12-dodecanediol, 1,13-tridecanediol, 1,14-tetradecanediol, 1,15-heptadecanediol, 1,16-hexadecanediol, 1,17-heptadecanediol, 1,18-octadecanediol, 1,19-nonadecanediol, 1,20-eicosadecanediol; (alkyl)cycloalkanediols (in which the alkyl group may be linear or branched, and the alkyl group and the hydroxyl group may be in any desired position) such as cyclohexanediol, methylcyclohexanediol; dihydric (alkyl)aryl alcohols having from 2 to 40 carbon atoms (in which the alkyl group may be linear or branched, and the alkyl group and the hydroxyl group may be in any desired position) such as benzenediol (catechol, etc.), methylbenzenediol, ethylbenzenediol, butylbenzenediol (p-tert-butylcatechol, etc.) dibutylbenzenediol (4,6-di-tert-butylresorcinol, etc.), 4,4′-thiobis(3-methyl-6-tert-butylphenol), 4,4′-butylidenebis(3-methyl-6-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-tert-butylphenol), 2,2′-thiobis(4,6-di-tert-butylresorcinol), 2,2′-methylenebis(4-ethyl-6-tert-butylphenol), 4,4′-methylenebis(2,6-di-tert-butylphenol), 2,2′-(3,5-di-tert-butyl-4-hydroxy)propane, 4,4′-cyclohexylidenebis(2,6-di-tert-butylphenol); p-tert-butylphenol/formaldehyde condensate, p-tert-butylphenol/acetaldehyde condensate; and their mixtures.

Of those, preferred are ethylene glycol, propylene glycol, neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 2-methyl-2,4-pentanediol, 2-ethyl-2-methyl-1,3-propanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol and 1,12-dodecanediol, in that they may more effectively lower the friction between the sliding member coated with the hard carbon thin film and the sliding surface of any other material. In addition, high-molecular-weight hindered alcohols having a molecular weight of at least 300, preferably at least 400 such as 2,6-di-tert-butyl-4-(3,5-di-tert-butyl-4-hydroxybenzyl)phenyl alcohol are also preferred in that they are hardly volatile even at high temperatures (for example, under sliding condition in internal-combustion engines) and are highly resistant to heat, and they can well exhibit their friction-reducing effect and can impart excellent antioxidation stability to the lubricating oil.

Tri- and higher polyalcohols are concretely those having three or more hydroxyl groups. In general, tri- to deca-alcohols, preferably tri- to hexa-alcohols are used. Examples of these components are trimethylolalkanes such as glycerin, trimethylolethane, trimethylolpropane, trimethylolbutane; as well as erythritol, pentaerythritol, 1,2,4-butanetriol, 1,3,5-pentanetriol, 1,2,6-hexanetriol, 1,2,3,4-butanetetrol, sorbitol, adonitol, arabitol, xylitol, mannitol; and their polymers or condensates (e.g., glycerin dimers to octamers such as diglycerin, triglycerin, tetraglycerin; trimethylolpropane dimers to octamers such as ditrimethylolpropane; pentaerythritol dimers to tetramers such as dipentaerythritol; sorbitan; condensates such as sorbitol/glycerin condensate (including intramolecular condensates, intermolecular condensates, and self-condensates)).

Saccharides such as xylose, arabitol, ribose, rhamnose, glucose, fructose, galactose, mannose, sorbose, cellobiose, mannose, isomaltose, trehalose and sucrose are also usable.

Of those, more preferred are tri to hexa-alcohols such as glycerin, trimethylolalkanes (e.g., trimethylolethane, trimethylolpropane, trimethylolbutane), pentaerythritol, 1,2,4-butanetriol, 1,3,5-pentanetriol, 1,2,6-hexanetriol, 1,2,3,4-butanetetrol, sorbitol, sorbitan, sorbitol/glycerin condensate, adonitol, arabitol, xylitol, mannitol, and their mixtures; and even more preferred are glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, sorbitan and their mixtures. Especially preferred are polyalcohols having an oxygen content of at least 20%, preferably at least 30%, more preferably at least 40%. Polyalcohols that are higher than hexa-alcohols will too much increase the viscosity of the lubricating oil.

The above component 1.4 includes alkylene oxide adducts of alcohols selected from 1.1 to 1.3. Concretely, they are prepared by adding an alkylene oxide having from 2 to 6, preferably from 2 to 4 carbon atoms or its polymer or copolymer to the alcohol to thereby hydrocarbyletherify or hydrocarbylesterify the hydroxyl group of the alcohol. The alkylene oxide having from 2 to 6 carbon atoms includes ethylene oxide, propylene oxide, 1,2-epoxybutane (α-butylene oxide), 2,3-epoxybutane (β-butylene oxide), 1,2-epoxy-1-methylpropane, 1,2-epoxyheptane, 1,2-epoxyhexane. Of those, preferred are ethylene oxide, propylene oxide and butylene oxide, in that they are more effective for reducing friction; and more preferred are ethylene oxide and propylene oxide.

When two or more different types of alkylene oxides are used herein, the mode of oxyalkylene polymerization is not specifically defined, and the groups may be random-copolymerized or block-copolymerized. When an alkylene oxide is added to a polyalcohol having from 2 to 6 hydroxyl groups, it may be added to all the hydroxyl groups of the polyalcohol or may be added to a part of the hydroxyl groups thereof.

Examples of carboxylic acids are mentioned below.

2.1 Aliphatic monocarboxylic acids (fatty acids). 2.2 Aliphatic polycarboxylic acids. 2.3 Carbon-cyclic carboxylic acids. 2.4 Heterocyclic carboxylic acids. 2.5 Mixtures of two or more selected from the above 2.1 to 2.4.

Aliphatic monocarboxylic acids (fatty acids) are concretely those having one carboxyl group in the molecule, including, for example, saturated aliphatic monocarboxylic acids having from 1 to 40 carbon atoms (in which the saturated aliphatic structure may be linear or branched) such as methanoic acid, ethanoic acid (acetic acid), propanoic acid (propionic acid), butanoic acid (butyric acid, isobutyric acid, etc.), pentanoic acid (valeric acid, isovaleric acid, pivalic acid, etc.), hexanoic acid (caproic acid, etc.), heptanoic acid, octanoic acid (caprylic acid, etc.), nonanoic acid (pelargonic acid, etc.), decanoic acid, undecanoic acid, dodecanoic acid (lauric acid, etc.), tridecanoic acid, tetradecanoic acid (myristic acid, etc.), pentadecanoic acid, hexadecanoic acid (palmitic acid, etc.), heptadecanoic acid, octadecanoic acid (stearic acid, etc.), nonadecanoic acid, eicosanoic acid, heneicosanoic acid, docosanoic acid, tricosanoic acid, tetracosanoic acid, pentacosanoic acid, hexacosanoic acid, heptacosanoic acid, octacosanoic acid, nonacosanoic acid, triacontanoic acid; and unsaturated aliphatic monocarboxylic acids having from 1 to 40 carbon atoms (in which the unsaturated aliphatic structure may be linear or branched, and the unsaturated bond may be in any desired position) such as propenoic acid (acrylic acid, etc.), propynoic acid (propiolic acid, etc.), butenoic acid (methacrylic acid, crotonic acid, isocrotonic acid, etc.), pentenoic acid, hexenoic acid, heptenoic acid, octenoic acid, nonenoic acid, decenoic acid, undecenoic acid, dodecenoic acid, tridecenoic acid, tetradecenoic acid, pentadecenoic acid, hexadecenoic acid, heptadecenoic acid, octadecenoic acid (oleic acid, etc.), nonadecenoic acid, eicosenoic acid, heneicosenoic acid, docosenoic acid, tricosenoic acid, tetracosenoic acid, pentacosenoic acid, hexacosenoic acid, heptacosenoic acid, octacosenoic acid, nonacosenoic acid, triacontenoic acid.

Aliphatic polycarboxylic acids include saturated or unsaturated aliphatic dicarboxylic acids having from 2 to 40 carbon atoms (in which the saturated aliphatic or unsaturated aliphatic structure may be linear or branched, and the unsaturated bond may be in any desired position) such as ethane-diacid (oxalic acid), propane-diacid (malonic acid, etc.), butane-diacid (succinic acid, methylmalonic acid, etc.), pentane-diacid (glutaric acid, ethylmalonic acid, etc.), hexane-diacid (adipic acid, etc.), heptane-diacid (pimelic acid, etc.), octane-diacid (suberic acid, etc.), nonane-diacid (azelaic acid, etc.), decane-diacid (sebacic acid, etc.), propene-diacid, butene-diacid (maleic acid, fumaric acid, etc.), pentene-diacid (citraconic acid, mesaconic acid, etc.), hexene-diacid, heptene-diacid, octene-diacid, nonene-diacid, decene-diacid; saturated or unsaturated tricarboxylic acids (in which the saturated aliphatic or unsaturated aliphatic structure may be linear or branched, and the unsaturated bond may be in any desired position) such as propane-tricarboxylic acid, butane-tricarboxylic acid, pentane-tricarboxylic acid, hexane-tricarboxylic acid, heptane-tricarboxylic acid, octane-tricarboxylic acid, nonane-tricarboxylic acid, decane-tricarboxylic acid; and saturated or unsaturated tetracarboxylic acids (in which the saturated aliphatic or unsaturated aliphatic structure may be linear or branched, and the unsaturated bond may be in any desired position).

Carbon-cyclic carboxylic acids are concretely those having one or more carboxyl groups in the carbon-cyclic molecule, including, for example, naphthene ring-having, mono, di, tri or tetracarboxylic acids having from 3 to 40 carbon atoms (in which the alkyl or alkenyl group, if any therein, may be linear or branched, and the double bond, if any therein, may be in any desired position, and the number and the position of the substituents are not defined) such as cyclohexane-monocarboxylic acid, methylcyclohexane-monocarboxylic acid, ethylcyclohexane-monocarboxylic acid, propylcyclohexane-monocarboxylic acid, butylcyclohexane-monocarboxylic acid, pentylcycohexane-monocarboxylic acid, hexylcyclohexane-monocarboxylic acid, heptylcyclohexane-monocarboxylic acid, octylcyclohexane-monocarboxylic acid, cycloheptane-monocarboxylic acid, cyclooctane-monocarboxylic acid, trimethylcyclopentane-dicarboxylic acid (camphor acid, etc.); aromatic monocarboxylic acids having from 7 to 40 carbon atoms such as benzenecarboxylic acid (benzoic acid), methylbenzenecarboxylic acid (toluic acid, etc.), ethylbenzenecarboxylic acid, propylbenzenecarboxylic acid, benzenedicarboxylic acid (phthalic acid, isophthalic acid, terephthalic acid, etc.), benzenetricarboxylic acid (trimellitic acid, etc.), benzeneteracarboxylic acid (pyromellitic acid, etc.), naphthalenecarboxylic acid (naphthoic acid, etc.); mono, di, tri or tetracarboxylic acids having an aryl group with from 7 to 40 carbon atoms (in which the alkyl or alkenyl group, if any therein as a substituent, may be linear or branched and the double bound, if any therein, may be in any desired position, and the number and the position of the substituents are not defined) such as phenylpropanoic acid (hydroatropic acid), phenylpropenoic acid (atropic acid, cinnamic acid, etc.), salicylic acid, alkylsalicylic acid having one or more alkyl groups with from 1 to 30 carbon atoms.

Heterocyclic carboxylic acids are concretely those having one or more carboxyl groups in the molecule, including, for example, those having from 5 to 40 carbon atoms such as furanecarboxylic acid, thiophenecarboxylic acid, pyridinecarboxylic acid (nicotinic acid, isonicotinic acid, etc.).

Examples of ethers are mentioned below.

3.1 Saturated or unsaturated aliphatic ethers. 3.2 Aromatic ethers. 3.3 Cyclic ethers. 3.4 Mixtures of two or more selected from the above 3.1 to 3.3.

Aliphatic monoethers are concretely saturated or unsaturated aliphatic ethers having from 1 to 40 carbon atoms (in which the saturated or unsaturated aliphatic structure may be linear or branched, and the unsaturated bond may be in any desired position) such as dimethyl ether, diethyl ether, di-n-propyl ether, diisopropyl ether, dibutyl ether, diisobutyl ether, di-n-amyl ether, diisoamyl ether, dihexyl ether, diheptyl ether, dioctyl ether, dinonyl ether, didecyl ether, diundecyl ether, didodecyl ether, ditridecyl ether, ditetradecyl ether, dipentadecyl ether, dihexadecyl ether, diheptadecyl ether, dioctadecyl ether, dinonadecyl ether, dieicosyl ether, methyl ethyl ether, methyl n-propyl ether, methyl isopropyl ether, methyl isobutyl ether, methyl tert-butyl ether, methyl n-amyl ether, methyl isoamyl ether, ethyl n-propyl ether, ethyl isopropyl ether, ethyl isobutyl ether, ethyl tert-butyl ether, ethyl n-amyl ether, ethyl isoamyl ether, divinyl ether, diallyl ether, methyl vinyl ether, methyl allyl ether, ethyl vinyl ether, ethyl allyl ether.

Concretely, aromatic ethers include, for example, anisole, phenetole, phenyl ether, benzyl ether, phenyl benzyl ether, α-naphthyl ether, β-naphthyl ether, polyphenyl ether, perfluoroether; and these may have a saturated or unsaturated group (in which the saturated or unsaturated group may be linear or branched, and the unsaturated bond may be in any desired position, and the number and the position of the substituents are not defined). Preferably, these are liquid under the service condition thereof, especially at room temperature.

Concretely, cyclic ethers are those having from 2 to 40 carbon atoms, including, for example, ethylene oxide, propylene oxide, trimethylene oxide, tetrahydrofuran, tetrahydropyran, dioxane, glycidyl ether; and these may have a saturated or unsaturated group, a carbon ring, or a saturated or unsaturated aliphatic group-having carbon ring (in which the saturated or unsaturated group may be linear or branched, and the unsaturated bond may be in any desired position, and the number and the position of the substituents are not defined).

Examples of esters are mentioned below.

4.1 Esters of aliphatic monocarboxylic acids (fatty acids). 4.2 Esters of aliphatic polycarboxylic acids. 4.3 Esters of carbon-cyclic carboxylic acids. 4.4 Esters of heterocyclic carboxylic acids. 4.5 Alkylene oxide adducts of alcohols or esters. 4.6 Mixtures of any compounds selected from the above 4.1 to 4.5.

Esters of the above 4.1 to 4.5 may be complete esters in which the hydroxyl group and the carboxyl group are all esterified, or partial esters in which the hydroxyl group or the carboxyl group partially remains as such.

The component of the above 4.1 is an ester of one or more selected from the above-mentioned aliphatic monocarboxylic acids (fatty acids) and one or more selected from the above-mentioned mono, di, tri or higher polyalcohols, and it includes an ashless fatty acid ester friction modifier. The ashless fatty acid ester friction modifier is a fatty acid ester having a linear or branched hydrocarbon group with from 6 to 30, preferably from 8 to 24, more preferably from 10 to 20 carbon atoms, and it includes esters of fatty acids having such a hydrocarbon group and aliphatic mono alcohols or aliphatic polyalcohols. The fatty acids as referred to herein are aliphatic monocarboxylic acids. Concretely, preferred examples of the esters are glycerin monooleate, glycerin dioleate, sorbitan monooleate, sorbitan dioleate, and the like.

The component 4.1 except the ashless fatty acid ester friction modifier includes fatty acid esters having a linear or branched hydrocarbon group with from 1 to 5 or from 31 to 40 carbon atoms. Their examples are esters of fatty acids having such a hydrocarbon group and aliphatic monoalcohols or aliphatic polyalcohols.

Of the above, those having a kinematic viscosity at 100° C. of from 1 to 100 mm²/sec may be used for the base oil for lube oil, and are generally differentiated from the above-mentioned ashless fatty acid ester friction modifier. Their examples are polyol esters such as single esters or complex esters that are formed of tri or higher polyols having from 3 to 40, preferably from 4 to 18, more preferably from 4 to 12 carbon atoms, especially tri or higher polyols having a neopentyl structure, and one or more selected from monocarboxylic acids having from 1 to 40, preferably from 4 to 18, more preferably from 6 to 12 carbon atoms, and their mixtures, as well as their adducts with alkylene oxides added thereto, for example, trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol 2-ethylhexanoate, pentaerythritol pelargonate. These may be complete esters in which the hydroxyl group and the carboxyl group are all esterified, or partial esters in which the hydroxyl group or the carboxyl group partly remains as such. However, they are preferably complete esters. The hydroxyl value of the esters is generally at most 100 mg KOH/g, preferably at most 50 mg KOH/g, more preferably at most 10 mg KOH/g.

Preferably, the kinematic viscosity at 100° C. of the base oil for lube oil is from 2 to 60 mm²/sec, more preferably from 3 to 50 mm²/sec.

The component 4.2 is an ester of one or more selected from the above-mentioned aliphatic polycarboxylic acids and one or more selected from the above-mentioned mono, di, tri or higher polyalcohols. Its preferred examples are diesters of one or more polycarboxylic acid selected from dicarboxylic acids having from 2 to 40, preferably from 4 to 18, more preferably from 6 to 12 carbon atoms, and one or more selected from monoalcohols having from 4 to 40, preferably from 4 to 18, more preferably from 6 to 14, such as dibutyl maleate, ditridecyl glutamate, di-2-ethylhexyl adipate, diisodecyl adipate, ditridecyl adipate, di-2-ethylhexyl sebacate, and copolymers of these diesters (e.g., dibutyl maleate) and poly-α-olefins having from 4 to 16 carbon atoms; and esters of α-olefin adducts to acetic anhydride or the like, and alcohols having from 1 to 40 carbon atoms. Of the compounds, those having a kinematic viscosity at 100° C. of from 1 to 100 mm²/sec may be used for the base oil for the lubricating oil.

The component 4.3 is an ester of one or more selected from the above-mentioned carbon-cyclic carboxylic acids, and one or more selected from the above-mentioned mono, di, tri or higher polyalcohols. Its preferred examples are aromatic carboxylic esters such as phthalic acid esters, trimellitic acid esters, pyromellitic acid esters, salicylic acid esters. Of the esters, those having a kinematic viscosity at 100° C. of from 1 to 100 mm²/sec may be used for the base oil for the lubricating oil.

The component 4.4 is an ester of one or more selected from the above-mentioned heterocyclic carboxylic acids, and one or more selected from the above-mentioned mono, di, tri or higher polyalcohols. Of the esters, those having a kinematic viscosity at 100° C. of from 1 to 100 mm²/sec may be used for the base oil for the lubricating oil.

The component 4.5 includes esters prepared by adding an alkylene oxide to one or more selected from the above-mentioned mono, di, tri or higher polyalcohols followed by esterifying them; and adducts prepared by adding an alkylene oxide to the esters selected from the above-mentioned components 4.1 to 4.4. Of the compounds, those having a kinematic viscosity at 100° C. of from 1 to 100 mm²/sec may be used for the base oil for lube oil.

Derivatives of the above-mentioned organic oxygen-containing compounds concretely include, for example, those prepared by sulfidizing one selected from the above-mentioned alcohols, carboxylic acids, esters and ethers; those prepared by halogenating (fluorinating, chlorinating) it; its reaction products with acids, such as sulfuric acid, nitric acid, boric acid, phosphoric acid, or their esters or metal salts; and its reaction products with metals, metal-containing compounds or amine compounds.

Of those, preferred are reaction products of one or more selected from alcohols and carboxylic acids and their derivatives, with amine compounds (e.g., Mannich reaction products, acylated products, amides).

The amine compounds as referred to herein include ammonia, monoamines, diamines, and polyamines. More concretely, their examples are ammonia; alkylamines having an alkyl group with from 1 to 30 carbon atoms (in which the alkyl group may be linear or branched) such as methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, octylamine, nonylamine, decylamine, undecylamine, dodecylamine, tridecylamine, tetradecylamine, pentadecylamine, hexadecylamine, heptadecylamine, octadecylamine, stearylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, diheptylamine, dioctylamine, dinonylamine, didecylamine, diundecylamine, didodecylamine, ditridecylamine, ditetradecylamine, dipentadecylamine, dihexadecylamine, diheptadecylamine, dioctadecylamine, methylethylamine, methylpropylamine, methylbutylamine, ethylpropylamine, ethylbutylamine, and propylbutylamine; alkenylamines having an alkenyl group with from 2 to 30 carbon atoms (in which the alkenyl group may be linear or branched) such as ethenylamine, propenylamine, butenylamine, octenylamine, and oleylamine; alkanolamines having an alkanol group with from 1 to 30 carbon atoms (in which the alnanol group may be linear or branched) such as methanolamine, ethanolamine, propanolamine, butanolamine, pentanolamine, hexanolamine, heptanolamine, octanolamine, nonanolamine, methanolethanolamine, methanolpropanolamine, methanolbutanolamine, ethanolpropanolamine, ethanolbutanolamine, and propanolbutanolamine; alkylenediamines having an alkylene group with from 1 to 30 carbon atoms, such as methylenediamine, ethylenediamine, propylenediamine, and butylenediamine; polyamines such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine; compounds derived from the above-mentioned monoamines, diamines or polyamines and further having an alkyl or alkenyl group with from 8 to 20 carbon atoms, such as undecyldiethylamine, undecyldiethanolamine, dodecyldipropanolamine, oleyldiethanolamine, oleylpropylenediamine, stearyltetraethylenepentamine; heterocyclic compounds such as N-hydroxyethyloleylimidazoline; alkylene oxide adducts of these compounds; and their mixtures.

Of those nitrogen-containing compounds, preferred are aliphatic amines having an alkyl or alkenyl group with from 10 to 20 carbon atoms (these may be linear or branched) such as decylamine, dodecylamine, tridecylamine, heptadecylamine, octadecylamine, oleylamine and stearylamine.

Of the derivatives of these oxygen-containing organic compounds, preferred are carbonamides having from 8 to 20 carbon atoms such as oleamide.

Examples of the aliphatic amine compound are aliphatic amine compounds each having C₆-C₃₀ straight or branched hydrocarbon chains or groups, preferably C₈-C₂₄ straight or branched hydrocarbon chains, more preferably C₁₀-C₂₀ straight or branched hydrocarbon chains. When the carbon number of the hydrocarbon chain is not within the range of 6 to 30, there arises a possibility that the lubricating oil may not produce a sufficient friction reducing effect as expected.

Specific examples of the C₆-C₃₀ straight or branched hydrocarbon chain include: alkyl groups, such as hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl and triacontyl; and alkenyl groups, such as hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl, octadecenyl, nonadecenyl, icosenyl, heneicosenyl, docosenyl, tricosenyl, tetracosenyl, pentacosenyl, hexacosenyl, heptacosenyl, octacosenyl, nonacosenyl and triacontenyl. The above alkyl and alkenyl groups include all possible isomers. Additionally, the position of alkenyl groups is free.

The aliphatic amine compound can be exemplified by aliphatic monoamines and alkylene oxide adducts thereof, aliphatic polyamines, imidazoline compounds, and derivatives thereof. Specific examples of such aliphatic amines include: aliphatic amine compounds, such as laurylamine, lauryldiethylamine, lauryldiethanolamine, dodecyldipropanolamine, palmitylamine, stearylamine, stearyltetraethylenepentamine, oleylamine, oleylpropylenediamine, oleyldiethanolamine and N-hydroxyethyloleylimidazolyne; adducts of the above aliphatic amines (C₆-C₂₈ alkyl or alkenyl amines) with alkylene oxides, such as N,N-dipolyoxyalkylene-N-alkylamines; and acid-modified compounds prepared by reacting the above aliphatic amines with C₂-C₃₀ monocarboxylic acids (such as fatty acids) or C₂-C₃₀ polycarboxylic acids (such as oxalic acid, phthalic acid, trimellitic acid and pyromellitic acid) so as to neutralize or amidate the whole or part of the remaining amino and/or imino groups. In connection with the present invention, N,N-dipolyoxyethylene-N-oleylamine is preferably used.

The amount (content) of the aliphatic amine compound contained in the lubricating oil for the manual transmission is not particularly restricted, and is preferably 0.05 to 3.0%, more preferably 0.1 to 2.0%, and most preferably 0.5 to 1.4%, based on the total mass of the lubricating oil. When the amount of the fatty acid ester friction modifier and/or the aliphatic amine friction modifier is less than 0.05%, there arises a possibility that the lubricating oil may not produce a sufficient friction reducing effect. When the amount of the fatty acid ester friction modifier and/or the aliphatic amine friction modifier exceeds 3.0%, the lubricating oil produce a good friction reducing effect but undesirably deteriorates in storage stability and compatibility to cause precipitations.

Further, the continuously variable transmission oil or lubricating oil for the continuously variable transmission, used in the present invention preferably includes polybutenyl succinimide and/or a derivative thereof as an ashless dispersant. Specific examples of the polybutenyl succinimide usable in connection with the present invention include compounds represented by the following general formulas (1) and (2).

In each of the formulas (1) and (2), n represents an integer of 1 to 5, preferably 2 to 4, so as to attain a good detergent effect. Further, PIB represents a polybutenyl group derived from polybutene. The polybutene can be prepared by polymerizing high-purity isobutene or a mixture of 1-butene and isobutene in the presence of a boron fluoride catalyst or an aluminum chloride catalyst in such a manner that the polybutene attains a number-average molecular weight of 900 to 3,500, preferably 1,000 to 2,000. When the number-average molecular weight of the polybutene is less than 900, there is a possibility of failing to attain a sufficient detergent effect. When the number-average molecular weight of the polybutene exceeds 3,500, the polybutene may undesirably deteriorate in low-temperature fluidity. In the production of the polybutenyl succinimide, the polybutene may be used after purified by removing trace amounts of fluorine and chlorine residues, which result from the above polybutene production catalyst, by any suitable treatment (such as adsorption process or washing process). The amount of the fluorine and chlorine residues is preferably controlled to 50 ppm or less, more preferably 10 ppm or less, most preferably 1 ppm or less.

The production method of the polybutenyl succinimide is not particularly restricted. For example, the polybutenyl succinimide can be prepared by reacting an chloride of the above-mentioned polybutene, or the polybutene from which fluorine and chlorine residues are removed, with maleic anhydride at 100 to 200° C. to form polybutenyl succinate, and then, reacting the thus-formed polybutenyl succinate with polyamine (such as diethylene triamine, triethylene tetramine, tetraethylene pentamine or pentaethylene hexamine).

The polybutenyl succinimide derivative can be exemplified by boron- and acid-modified compounds obtained by reacting the polybutenyl succinimide of the formulas (1) and (2) with boron compounds or oxygen-containing organic compounds so as to neutralize or amidate the whole or part of the remaining amino and/or imide groups. Among these, boron-containing polybutenyl succinimide, especially boron-containing bis(polybutenyl)succinimide, is preferably used.

The above boron compound can be a boric acid, a borate or a boric acid ester. Specific examples of the boric acid include orthoboric acid, metaboric acid and paraboric acid. Specific examples of the borate include: ammonium salts including ammonium borates, such as ammonium metaborate, ammonium tetraborate, ammonium pentaborate and ammonium octaborate. Specific examples of the boric acid ester include: esters of boric acids and alkylalcohols (preferably C₁-C₆ alkylalcohols), such as monomethyl borate, dimethyl borate, trimethyl borate, monoethyl borate, diethyl borate, triethyl borate, monopropyl borate, dipropyl borate, tripropyl borate, monobutyl borate, dibutyl borate and tributyl borate. Here, the content ratio of nitrogen to boron (B/N) by mass in the boron-containing polybutenyl succinimide is usually 0.1 to 3, preferably 0.2 to 1.

The above organic oxygen-containing compound can be exemplified by: C₁-C₃₀ monocarboxylic acids, such as formic acid, acetic acid, glycolic acid, propionic acid, lactic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, undecylic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, oleic acid, nonadecanoic acid and eicosanoic acid; C₂-C₃₀ polycarboxylic acids, such as oxalic acid, phthalic acid, trimellitic acid and pyromellitic acid, and anhydrides and esters thereof; C₂-C₆ alkylene oxides; and hydroxy(poly)oxyalkylene carbonates.

The amount of the polybutenyl succinimide and/or the derivative thereof added in the lubricating oil is not particularly restricted, and is preferably 0.1 to 15%, more preferably 1.0 to 12%, based on the total mass of the lubricating oil. When the amount of the polybutenyl succineimide and/or the derivative thereof is less than 0.1%, there arises a possibility of failing to attain a sufficient detergent effect. It becomes uneconomical when the amount of the polybutenyl succineimide and/or the derivative thereof exceeds 15%. In addition, such a large amount of the polybutenyl succineimide and/or the derivative thereof tends to cause a deterioration in demulsification ability.

As discussed above, in connection with the present invention, the lubricating oil can exhibit an extremely excellent low friction characteristics in case of being used between the sliding surface of the hard carbon thin film such as the DLC film and the opposite member. In order to raise performances required for the lubricating oil (composition) for the continuously variable transmission, the lubricating oil may contain other additives, such as a metallic detergent, an antioxidant, a viscosity index improver, a friction modifier other than the above-mentioned fatty acid ester friction modifier and/or the aliphatic amine friction modifier, an ashless dispersant other than the above-mentioned polybutenyl succinimide and/or the derivative thereof, an anti-wear agent or extreme-pressure additive (agent), a rust inhibitor, a nonionic surfactant, a deemulsifier, a metal deactivator and/or an anti-foaming agent, when used in an internal combustion engine. These additives may be used alone or in the form of a mixture of two or more thereof so as to meet the lubricating oil performance required.

The metallic detergent can be any metallic-detergent compound commonly used for a lubricating oil. Specific examples of the metallic detergent usable in connection with the present invention include sulfonates, phenates and salicylates of alkali metals or alkali-earth metals; and mixtures of two or more thereof. Examples of the alkali metals include sodium (Na) and potassium (K), and examples of the alkali-earth metals include calcium (Ca) and magnesium (Mg). In connection with the present invention, sodium and calcium sulfonates, sodium and calcium phenates, and sodium and calcium salicylates are suitably used. The total base number and amount of the metallic detergent can be selected in accordance with the lubricating oil performance required. The total base number of the metallic detergent is usually 0 to 500 mgKOH/g, preferably 150 to 400 mgKOH/g, as measured by perchloric acid method according to ISO 3771 “Determination of base number—Perchloric acid potentiometric titration method”. The amount of the metallic detergent is usually 0.1 to 10% based on the total mass of the lubricating oil.

The antioxidant can be any antioxidant compound commonly used for a lubricating oil. Specific examples of the antioxidant usable in connection with the present invention include: phenolic antioxidants, such as 4,4′-methylenebis(2,6-di-tert-butylphenol) and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; amino antioxidants, such as phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine and alkyldiphenylamine; and mixtures of two or more thereof. The amount of the antioxidant is usually 0.01 to 5% based on the total mass of the lubricating oil.

The viscosity index improver can be exemplified by: non-dispersion type viscosity index improvers, such as copolymers of one or two monomers selected from various methacrylic acids, and hydrides of the copolymers; and dispersion type viscosity index improvers, such as copolymers of methacrylates (including nitrogen compounds). There may be also used, as the viscosity index improver, copolymers of ethylene and α-olefins (such as propylene, 1-butene and 1-pentene) and hydrides thereof, polyisobutylenes and hydrides thereof, a hydrogenated copolymer of styrene and diene, a copolymer of styrene and maleic anhydride and polyalkylstyrenes.

The molecular weight of the viscosity index improver needs to be selected in view of shear stability. For example, the number-average molecular weight of the viscosity index improver is desirably in a range of 5,000 to 1,000,000, more desirably 100,000 to 800,000, for dispersion or non-dispersion type polymethacrylates; in a range of 800 to 5,000 for polyisobutylenes and hydrides thereof; and in a range of 800 to 300,000, more desirably 10,000 to 200,000 for ethylene/α-olefin copolymers and hydrides thereof. The above viscosity index improving compounds can be used alone or in the form of a mixture of two or more thereof. The amount of the viscosity index improver is preferably 0.1 to 40.0 based on the total mass of the lubricating oil.

The friction modifier other than the above-mentioned the aliphatic amine compound can be exemplified by ashless friction modifiers, such as boric acid esters, higher alcohols and aliphatic ethers, and metallic friction modifiers, such as molybdenum dithiophosphate, molybdenum dithiocarbamate and molybdenum disulfide.

The ashless dispersant other than the above-mentioned polybutenyl succinimide and/or the derivative thereof can be exemplified by polybutenylbenzylamines and polybutenylamines each having polybutenyl groups of number-average molecular weight of 900 to 3,500, polybutenyl succinimides having polybutenyl groups of number-average molecular weight of less than 900 and derivatives thereof.

The anti-friction agent or extreme-pressure additive can be exemplified by disulfides, sulfurized fats and oils, olefin sulfides, phosphate esters having one to three C₂-C₂₀ hydrocarbon groups, thiophosphate esters, phosphite esters, thiophosphite esters and amine salts of these esters.

The rust inhibitor can be exemplified by alkylbenzene sulfonates, dinonylnaphthalene sulfonates, esters of alkenylsuccinic acids and esters of polyhydric alcohols.

The nonionic surfactant and the deemulsifier can be exemplified by noionic polyalkylene glycol surfactants, such as polyoxyethylene alkylethers, polyoxyethylene alkylphenyleters and polyoxyethylene alkylnaphthyleters.

The metal deactivator can be exemplified by imidazoline compounds, pyrimidine derivatives, thiazole and benzotriazole. The anti-foaming agent can be exemplified by silicones, fluorosilicones and fluoroalkylethers.

Each of the friction modifier other than the fatty acid ester friction modifier and/or the aliphatic amine friction modifier, the ashless dispersant other than the polybutenyl succinimide and/or the derivative thereof, the anti-wear agent or extreme-pressure additive, the rust inhibitor and the demulsifier is usually contained in an amount of 0.01 to 5% based on the total mass of the lubricating oil, and the metal deactivator is contained in an amount of 0.0005 to 1% based on the total mass of the lubricating oil.

Experiment 1

The invention is described more concretely with reference to the following Examples and Comparative Examples, which, however, are not intended to restrict the scope of the invention.

[Fabrication of Sliding Members]

A carburized SCM420 steel was used as a base material. This was formed into blocks and rings of the sizes defined by Falex Corporation for LFW-1 test (see “LFW-1 Test Condition” mentioned below).

Block:

Blocks of SCM420 steel were carburized and tempered at a low temperature, and their sliding face was polished and finished with a wrapping tape to have a surface roughness Ra of 0.03 μm. The surface of the thus-processed blocks was optionally processed by PVD or CVD to be coated with a coating material so as to form thereon a coating layer (hard carbon thin film) having a thickness of 1.1 μm, followed by being further polished and finished with a wrapping tape to have a surface roughness Ra of 0.03 μm.

Ring:

Rings of SCM420 steel were carburized and tempered at a low temperature, and their sliding face was polished and finished with a wrapping tape to have a surface roughness Ra of 0.04 μm. The details of the blocks and the rings thus prepared are summarized in Table 1.

TABLE 1 Ring Block Kinds of Base Surface Surface Base Surface Surface Surface Surface sliding material Roughness Hardness material roughness hardness Coating Thickness, roughness hardness members (substrate) Ra, μm HRC (substrate) Ra, μm HRC material μm Ra, μm Hv A SCM420 0.04 59 SCM420 0.03 59 a-C 1.1 0.03 1850 carburized carburized steel steel B SCM420 0.04 59 SCM420 0.03 59 DLC 1.1 0.03 1850 carburized carburized (a-C:H) steel steel C SCM420 0.04 59 SCM420 0.03 59 none — — — carburized carburized steel steel Note: HRC indicates Rockwell hardness (C scale).

[Preparation of CVTF Composition]

A paraffin-based mineral oil (having a kinematic viscosity at 100° C. of 4.3 mm²/sec) was prepared as a base oil. A fatty acid ester, aliphatic amine, amide or alcohol friction modifier, and CVTF package additive and polymethacrylate were suitably added to it to prepare a CVTF composition. The resulting CVTF composition was combined with the above-mentioned sliding parts as in Examples 1-1 to 1-11 and Comparative Examples 1-1 and 1-2. The details of these Examples and Comparative Examples are shown in Table 2.

In Table 2, the fatty acid ester is glycerin monooleate; the commercial product CVTF is Nissan genuine oil “NS-2”, and this CVTF contains a friction modifier; and the CVTF package additive contains a mixture of detergent dispersant, antioxidant, anti-wear agent, anti-corrosive agent, pour point depressant, defoaming agent, and the like (but does not contain a friction modifier).

TABLE 2 Comparative Example Example 1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 1-9 1-10 1-11 1-1 1-2 Sliding members A A A A A A A B B B B B C Base oil (%) 79.3 79.3 79.3 79.3 80 78.5 (commer- 79.3 79.3 79.3 79.3 80 79.3 Fatty acid Ester (%) 0.7 — — — — 1.5 cial 0.7 — — — — 0.7 Aliphatic amine (%) — 0.7 — — — — product, — 0.7 — — — — Amide (%) — — 0.7 — — — CVTF) — — 0.7 — — — Alcohol (%) — — — 0.7 — — — — — 0.7 — — CVTF Package Additive 10 10 10 10 10 10 10 10 10 10 10 10 (%) Polymethacrylate (%) 10 10 10 10 10 10 10 10 10 10 10 10 Friction coefficient 0.035 0.04 0.04 0.04 0.07 0.03 0.035 0.09 0.1 0.1 0.1 0.13 0.12

[Performance Evaluation]

According to the LFW-1 test method defined in ASTM D2714, the samples were tested under the test condition mentioned below. From the friction force of each sample (the block) in 60 minutes after the start of the test, the friction coefficient thereof was calculated. The results obtained are shown in Table 4.

LFW-1 Test Condition:

Test Device: Falex Block-on-Ring Friction and Wear Testing Machine produced by Falex corporation;

Test Piece:

-   -   Block (Falex Type H-60)     -   Ring (Falex Type S-10);

Sliding Velocity: 0.26 m/sec;

Load: 222.5 N;

Oil Temperature: 110° C.; and

Test Period: 60 minutes.

From Table 2, it will be understood that the combination of the friction modifier and DLC (a-C) brings about a great friction-reducing effect as compared with the case of DLC (a-C:H) alone or the case with no coating. At present, it is believed that Example 3-6 will be the best in point of the friction-reducing effect thereof.

As described hereinabove, in this embodiment, a certain continuously variable transmission oil composition is present in the site between the sliding member coated with the hard carbon thin film and an opposite member formed of any material. Accordingly, this embodiment provides a low-friction sliding member in continuously variable transmissions, the advantages of which are that its friction is extremely low and stable and it has good wear resistance, providing a non-stage transmission oil composition for it.

Second Embodiment

A second embodiment of a low-friction sliding member in a transmission, according to the present invention will be discussed.

A low-friction sliding member in an automatic transmission is at least one of sliding members which are in slidable contact with each other in presence of a continuously variable transmission oil or fluid (composition) which may be referred to as “ATF (composition)”. The low-friction sliding member has a sliding section whose sliding surface is coated with a hard carbon thin film. The automatic transmission oil (lubricating oil) contains at least one of organic oxygen-containing compound and aliphatic amine compound as ashless friction modifier(s). By virtue of this automatic transmission oil, at least one of the sliding surfaces of the sliding members is coated with the hard carbon thin film, and therefore the sliding members can be in slidable contact with each other with an extremely low friction as compared with that in conventional techniques. Examples of the low-friction sliding member are gears, rotatable shafts, parts of bearings, and the like in the continuously variable transmission. FIG. 2 shows a part of a tooth 10A of a gear used in the automatic transmission. The gear tooth 10A includes the base material 12A having the sliding surface which is coated with the hard carbon thin film 14A.

The hard carbon thin film is formed of, for example, DLC (diamond-like carbon) material which is mainly constituted of carbon atom and amorphous. The DLC material takes a diamond structure (SP³ bonding) and/or a graphite structure (SP² bonding) in bonding mode among carbons. More specifically, the hard carbon (DLC) thin film 1 a is formed of hydrogen-free amorphous carbon (a-C) that consists of carbon, hydrogen-containing amorphous carbon (a-C:H), or metal carbide or metal carbon (MeC) that contains as a part a metal element of titanium (Ti) or Molybdenum (Mo).

As a hydrogen content in the hard carbon thin film increases, the friction coefficient increases. Accordingly, in the present invention, it is necessary that the upper limit of the hydrogen content in the hard carbon thin film is 20 atomic %. In order to sufficiently lower the friction coefficient in the lubricating oil during sliding and to obtain a stable sliding characteristics, the hydrogen content in the hard carbon thin film is preferably not more than 10 atomic %, and more preferably not more than 0.5 atomic %.

The hard carbon thin film having such a low hydrogen content is obtained by a PVD process that substantially does not use hydrogen and/or hydrogen-containing compound, such as a sputtering process or an ion plating process. In this case, it is preferable to carry out a film-forming operation for the hard carbon thin film upon baking of a reactor and tools for supporting the base material and upon sufficiently cleaning the surface of the base material in order to reduce the hydrogen content in the hard carbon thin film, in addition to using gas containing no hydrogen during the film-forming operation.

The hard carbon thin film is formed on the surface of a base material. Since the film thickness of the hard carbon thin film is considerably small, the surface roughness of the base material in a condition before the hard carbon thin film is formed largely affects the surface roughness of the hard carbon thin film formed on the surface of the base material. Accordingly, the surface roughness (center line average roughness) Ra of the base material is preferably not larger than 0.1 μm in the condition where the base material has not yet been coated with the hard carbon thin film. If the surface roughness of the base material exceeds 0.1 μm, projecting portions due to the surface roughness of the hard carbon thin film increases a local Hertz's contact pressure to an opposite member, thereby resulting in induction of formation of crack in the hard carbon thin film. The surface roughness Ra is explained as R_(a75) in JIS (Japanese Industrial Standard) B 0601 (:2001).

The hard carbon thin film preferably has a surface hardness or micro Vickers hardness (Hv) ranging from 1000 to 3500 as measured under application of a 10 g load and a thickness ranging from 0.3 to 2.0 μm. If the surface hardness and thickness of the hard carbon thin film are out of the above ranges, wear of the hard carbon thin film will occur in case that the surface hardness (Hv) is smaller than 1000 and the thickness is smaller than 0.3 μm, whereas peeling-off of the hard carbon thin film tends to occur in case that the surface hardness (Hv) exceeds 3500 and the thickness exceeds 2.0 μm.

The low-friction sliding member of this embodiment is particularly effective to be used as sliding members of an automatic transmission which sliding members have respective sliding surfaces which are in slidable contact with the sliding surfaces of the opposite members which are formed of an iron-based material or an aluminum alloy, in presence of the automatic transmission oil provided between the contactable sliding surfaces. The automatic transmission of this embodiment excludes the continuously variable transmission and therefore means a transmission in which speed-changing operation is discontinuously or stepwise made.

Next, the automatic transmission oil used for the continuously variable transmission will be discussed.

The automatic transmission oil preferably contains a base oil, and at least one of organic oxygen-containing compound and aliphatic amine compound as ashless friction modifier(s). It is to be noted that an extremely excellent low friction characteristics can be obtained in presence of the transmission oil at the sliding surface coated with the hard carbon thin film.

The base oil, the organic oxygen-containing compound and the aliphatic amine compound of the automatic transmission oil are the same as those used in the continuously variable transmission oil in connection with the first embodiment. Additionally, the automatic transmission oil in this embodiment may contain other components than the base oil, the organic oxygen-containing compound and the aliphatic amine compound, used in the continuously variable transmission oil.

Experiment 2

The invention is described more concretely with reference to the following Examples and Comparative Examples, which, however, are not intended to restrict the scope of the invention.

[Fabrication of Sliding Members]

A carburized SCM420 steel was used as a base material. This was formed into blocks and rings of the sizes defined by Falex Corporation for LFW-1 test (see “LFW-1 Test Condition” mentioned below).

Block:

Blocks of SCM420 steel were carburized and tempered at a low temperature, and their sliding face was polished and finished with a wrapping tape to have a surface roughness Ra of 0.03 μm. The surface of the thus-processed blocks was optionally processed by PVD or CVD to be coated with a coating material so as to form thereon a coating layer (hard carbon thin film) having a thickness of 1.1 μm, followed by being further polished and finished with a wrapping tape to have a surface roughness Ra of 0.03 μm.

Ring:

Rings of SCM420 steel were carburized and tempered at a low temperature, and their sliding face was polished and finished with a wrapping tape to have a surface roughness Ra of 0.04 μm. The details of the blocks and the rings thus prepared are the same as those summarized in Table 3

TABLE 3 Ring Block Kinds of Base Surface Surface Base Surface Surface Surface Surface sliding material Roughness Hardness material roughness hardness Coating Thickness, roughness hardness members (substrate) Ra, μm HRC (substrate) Ra, μm HRC material μm Ra, μm Hv A1 SCM420 0.04 59 SCM420 0.03 59 a-C 1.1 0.03 1850 carburized carburized steel steel B1 SCM420 0.04 59 SCM420 0.03 59 DLC 1.1 0.03 1850 carburized carburized (a-C:H) steel steel C1 SCM420 0.04 59 SCM420 0.03 59 none — — — carburized carburized steel steel Note: HRC indicates Rockwell hardness (C-scale).

[Preparation of ATF Composition]

A paraffin-based mineral oil (having a kinematic viscosity at 100° C. of 4.3 mm²/sec) was prepared as a base oil. A fatty acid ester, aliphatic amine, amide or alcohol friction modifier, and ATF package additive and polymethacrylate were suitably added to it to prepare a ATF composition. The resulting ATF composition was combined with the above-mentioned sliding members as in Examples 2-1 to 2-11 and Comparative Examples 2-1 and 2-2. The details of these Examples and Comparative Examples are shown in Table 5.

In Table 4, the fatty acid ester is glycerin monooleate; the commercial product ATF is Nissan genuine oil “Matic-J”, and this ATF contains a friction modifier; and the ATF package additive contains a friction modifier.

TABLE 4 Comparative Example example 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10 2-11 2-12 2-1 Sliding members A1 A1 A1 A1 A1 A1 A1 B1 B1 B1 B1 B1 C1 Base oil (%) 79.3 79.3 79.3 79.3 80 78.5 (commer- 79.3 79.3 79.3 79.3 80 79.3 Fatty acid ester (%) 0.7 — — — — 1.5 cial 0.7 — — — — 0.7 Aliphatic amine (%) — 0.7 — — — — product, — 0.7 — — — — Amide (%) — — 0.7 — — — ATF) — — 0.7 — — — Alcohol (%) — — — 0.7 — — — — — 0.7 — — ATF package additive 10 10 10 10 10 10 10 10 10 10 10 10 (%) Polymethacrylate (%) 10 10 10 10 10 10 10 10 10 10 10 10 Friction coefficient 0.035 0.04 0.04 0.035 0.04 0.03 0.035 0.07 0.08 0.08 0.08 0.11 0.1

[Performance Evaluation]

According to the LFW-1 test method defined in ASTM D2714, the samples were tested under the test condition mentioned below. From the friction force of each sample (the block) in 60 minutes after the start of the test, the friction coefficient thereof was calculated. The results obtained are shown in Table 4.

LFW-1 Test Condition:

Test Device: Falex Block-on-Ring Friction and Wear Testing Machine produced by Falex Corporation;

Test Piece:

-   -   Block (Falex Type H-60)     -   Ring (Falex Type S-10);

Sliding Velocity: 0.26 m/sec;

Load: 222.5 N;

Oil Temperature: 110° C.; and

Test Period: 60 minutes.

From Table 4, it is understood that the combination of the friction modifier and DLC (a-C) brings about a great friction-reducing effect as compared with the case of DLC (a-C:H) alone or the case with no coating. At present, it is believed that Example 2-6 will be the best in point of the friction-reducing effect thereof.

As described hereinabove, in this embodiment, a certain automatic transmission oil composition is present in the site between the sliding member coated with the hard carbon thin film and an opposite member formed of any material. Accordingly, this embodiment provides a low-friction sliding member in continuously variable transmissions, the advantages of which are that its friction is extremely low and stable and it has good wear resistance, providing an automatic transmission oil composition for it.

While the hard carbon thin film (for example, of the diamond-like carbon) has been described as being formed or obtained by the PVD process in the embodiments, it will be understood that the hard carbon thin film may be formed by a CVD process.

Although the invention has been described above by reference to certain embodiments and examples of the invention, the invention is not limited to the embodiments and examples described above. Modifications and variations of the embodiments and examples described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims. 

1. A low-friction sliding member in a transmission, the sliding member being at least one of a plurality of sliding members which are in slidable contact with each other in the presence of a transmission oil composition, the low-friction sliding member comprising: a base material having a sliding section; and a hard carbon thin film coated at a sliding surface of the sliding section of the base material, the hard carbon thin film containing hydrogen in an amount not more than 10 atomic %, wherein the transmission oil composition contains at least one of organic oxygen-containing compound and aliphatic amine compound.
 2. A low-friction sliding member as claimed in claim 1, wherein the transmission comprises a continuously variable transmission.
 3. A low-friction sliding member as claimed in claim 2, wherein the hard carbon thin film contains hydrogen in an amount not more than 0.5 atomic %.
 4. A low-friction sliding member as claimed in claim 2, wherein the sliding section of the base material has a surface roughness (Ra) not larger than 0.1μ in a condition before the base material is coated with the hard carbon thin film.
 5. A low-friction sliding member as claimed in claim 2, wherein the hard carbon thin film has a surface hardness (Hv) ranging from 1000 to 3500 in micro Vickers hardness as measured under application of a 10 g load, and a thickness ranging from 0.3 to 2.0 μm.
 6. A low-friction sliding member as claimed in claim 1, wherein the transmission comprises an automatic transmission.
 7. A low-friction sliding member as claimed in claim 6, wherein the hard carbon thin film contains hydrogen in an amount not more than 0.5 atomic %.
 8. A low-friction sliding member as claimed in claim 6, wherein the sliding section of the base material has a surface roughness (Ra) not larger than 0.1μ in a condition before the base material is coated with the hard carbon thin film.
 9. A low-friction sliding member as claimed in claim 6, wherein the hard carbon thin film has a surface hardness (Hv) ranging from 1000 to 3500 in micro Vickers hardness as measured under application of a log load, and a thickness ranging from 0.3 to 2.0 μm.
 10. A low-friction sliding member as claimed in claim 2, wherein the hard carbon thin film comprises amorphous DLC and is formed by a PVD process.
 11. A low-friction sliding member as claimed in claim 6, wherein the hard carbon thin film comprises amorphous DLC and is formed by a PVD process.
 12. A low-friction sliding member in a transmission, the sliding member being at least one of a plurality of sliding members which are in slidable contact with each other in the presence of a transmission oil composition, the low-friction sliding member comprising: a base material having a sliding section; and a hard carbon thin film coated at a sliding surface of the sliding section of the base material, the hard carbon thin film containing hydrogen in an amount not more than 20 atomic %, the hard carbon thin film being formed by a PVD process, wherein the transmission oil composition contains at least one of organic oxygen-containing compound and aliphatic amine compound. 